With just a thin long sleeve shirt underneath would this coat keep me warm in 0 to -10 degree weather. Also, what two coats would this one be in between in degrees of puffiness.

In this jacket I am comfortable down to about 20 degrees F with a light baselayer underneath. This is a little bit warmer than the Patagonia Down Sweater.

I own both the Marmot Zeus and Mountain Hardware Phantom. If I were knowingly going to encounter the temps you're describing I would go with the Phantom. The Zeus is great down to about 30ish, but below that for extended exposures its just not enough. You could pack on layers with the Zeus but at a point you'd be defeating the purpose of down. Down breathes and with light layering you get all the heat and no humidity, so you feel toasty, not hot. Down is super versatile, but you gotta respect the limits of the design. I love my Zeus and living in KY it gets more wear than the Phantom. Its light, comfortable over a harness for belaying, its got a very active fit and its stylish. Highly recommended, just not for the temps your describing.

Obviously different people will be comfortable at different temperatures in this. However, I would not plan on this being warm down to the temperatures that you are looking at. I have a Patagonia down sweater, and underneath a shell with a mid-weight baselayer I am comfortable skiing down into the low to mid 20s. The Zeus is probably a little bit warmer than the patagonia down sweater (it seemed to be a little puffier when I compared them side by side - might be the larger baffles though), but I doubt it is significantly warmer. This is the lightest down coat that Marmot makes, so you'd have to look at something like the Patagonia down sweater or the Mountain Hardwear Nitrous jacket should be less puffy. The Mountain Hardwear Phantom jacket should be a little bit more puffy, and the Marmot Guides Down Sweater will be substantially puffier.